The attack is the latest in a string of assaults on teams grappling with a near nine-month-old Ebola outbreak that has claimed almost 850 lives.

A health worker fighting an Ebola outbreak in DR Congo was killed Friday in an attack on a hospital in the eastern city of Butembo, the World Health Organization (WHO) said.

The attack is the latest in a string of assaults on teams grappling with a near nine-month-old Ebola outbreak that has claimed almost 850 lives.

"The @WHO family lost a dear colleague in the hospital attack in Butembo, #DRC, today," WHO chief Tedros Adhanom Ghebreyesus said in a tweet.

"My colleagues and I are grieving over the loss of this courageous colleague who was saving lives to end #Ebola. We are outraged by the attack. Health workers are #NotATarget."

Police said the casualty was a Cameroonian doctor who had been taking part in a meeting of an anti-Ebola team when the hospital was attacked by a rebel group.

DR Congo declared a tenth outbreak of Ebola in 40 years last August in northeastern North Kivu province before the virus spread into the neighbouring Ituri region.

The epicentre of the outbreak was first located in the rural area of Mangina, but then switched to the town of Beni.

Local organisations say the number of Ebola deaths is rising.

An updated toll by the health ministry, issued on Wednesday, said there had been 843 deaths.

WHO data from April 9 put the number of confirmed or probable cases at 1,186, of which 751 had been fatal.

The outbreak is the second deadliest on record, after the epidemic that struck West Africa in 2014-16, which killed more than 11,300 people.

The effort to roll back the disease has been hampered by fighting but also by resistance within communities to preventative measures, care facilities and safe burials.

On March 9, an attack on a treatment centre at Butemo left a policeman dead and a health worker wounded. It was the third attack on the centre, according to mayor Sylvain Kanyamanda Mbusa.

On February 24, a treatment centre in Katwa was set ablaze.

On April 12, the WHO's Emergency Committee, in a new review of the outbreak, determined that the situation did not constitute a "public health emergency of international concern," a status that initiates a major global response.